1. Field of the Invention
This invention relates to a process for generating a look-up table for use in displaying a highlight object on a raster display and, in particular, an invertible look-up table for use in displaying a cursor on a background of a raster display.
2. Description of Related Art
Nearly every interactive computer program must display a highlight object of one form or another, such as a cursor, on a background of a display. Two techniques are commonly used to allow the cursor to be visible.
One conventional technique is to use the exclusive--OR operator to replace a value in a display buffer with another value to make a cursor visible. This technique has the advantages of being very computationally efficient, and having perfect spatial resolution. The problem with this technique is that it can produce a very low visibility cursor in displays which are not binary since the result of inverting the bits of a color are unpredictable.
The second conventional technique is to read the contents of the display buffer, and store the contents of the area to be effected by the cursor, then write the cursor to the display replacing the background values with the cursor values. This cursor is visible by the nature of the values written. Typically, the cursor has a black body surrounded by a white "halo". This guarantees that either the white or black of the cursor will be visible on the background. This technique is even more computationally efficient than the first technique described, but suffers in spatial resolution. The difficulty is that if the cursor is on a white background, only the black portion is visible. If the cursor is on a black background, only the white "halo" is visible. This is a drawback in that the user cannot know precisely which pixel is being indicated with this cursor type.
An object of the present invention is to allow a cursor to be visible on any background without having to combine the cursor with the background, without sacrificing spatial resolution, and without having to first save the contents of the display for the area to be written with the cursor.
These and other objects of the invention will be clear from the following description.